Single fire enameling process and article



coat .is applied to the metal. ground coats, by reason of the inclusion of cobalt-or other adhesion promoting metal, are

i 'atenteci July 8, 1952 UNITED STATES PATENT OFFICE,

SINGLE FIRE ENAMELING PROCESQ AND ARTICLE Theodore F. 011; and Jesse .I .Canfield,- Middletown, tion, Middletown,

Ohio, assignors to Armco Steel Corpora- Ohio, a corporation of Ohio No Drawing. Application March 22, 1950, Serial'No. 151,309

This application is a continuation-in-part of our copending application, Serial No. 612,720, filed August 25, 1945, and entitled Single Fire :Enameling Process and Article, the said application being since abandoned; and the invention has to do with the problem of providing in a single firing an article having a commercial, smooth, continuous coating of vitreous or porcelain enamel of white or light color.

The solution of the problem involves two factors: (.15 the use of an enameling steel; or

. Certain .enameling stocks are available which are free from or have a greatly diminished tendency toward primary-boiling. Developments in white or light colored enamels having sufficient smoothness and opacity for commercial purposes are in: progress, and certain of these developments are directed toward enhancing the adhesion between such white or light colored enamel coatings and the base metal. The fact remains, however, that a. white or light colored vitreous enamel coating having fully satisfactory adhesion to the: usually'relatively smooth surfaces of vitreous enameling stocks and articles has not been commercially developed, and this factor has delayed the commercial application of single fire white or light colored vitreous enamel processes.

Consequently, in commercial practice the art has produced white or light colored enamelware by a procedure involving a plurality of firings in the first. of which. a. high adhesion ground High adhesion dark in color. Afterlthe article has cooled from 'the initial firing, a whiteor light colored frit is applied over the fused ground coat, and the article: isagain fired. The white or light colored enamel substance preferably has a firing temperature lower than that of the ground coat so that it may be fused'to cover the ground coat without such softening of the latter as would lead to diffusion-and the loss of the white orlight color'value ofthe overcoat. Under these circumstances the useof an enameling stock or '8 Claims. (01. lit-"'70) rerrous base which is free from the primary boiling difficulty is not necessary since the white or light colored overcoat may be so applied and fired as to mask any imperfections'existing in the ground coat. Then, too, the thickness or the conventional ground coatitself issuch' that it tends to mask boiling defects. v

A fundamental object of our inllention. is the provision of satisfactory single fired coatings of enamel, particularly in white or light colors, with concurrent solution of the problems of adhesion, opacity, acid resistance and the like. Another important object of the invention is the provision of improved white or light colored enamel coatings and of improved white or light colored enameled articles which, in particular, are superior to those hitherto known in the art in that they are thinner and hence have less tendency toward chipping, while preserving satisfactory opacity and adherence.

These and other objects or the inveiitiomwhich will beset forth hereinafter or will be apparent taught, to produce a satisfactory white or light commerce-lain enameled article in a single firing by the use of an exceedingly thin underlaye'r of a groundcoat enamel which develops high 'bondandan overlayer of white or light colored porcelain enamei, whilepreserving gloss, opacity and reflectivity, if certain steps" are taken amounting to a reversal of the techniques hitherto practiced and considered to be necessary by the art. n describing our invention, we shall take up in order'the two factors involved .in the problem.

In a copjend'ing application in our names entitled Enameling Stock, ,fild August 25, 1945, SerialNd. 512,721, the provision ofiron or mild steel shcleto'r strip stock or articles ,hasbeen taught, Where the stock is s'o'prjepared that it will receive" single fired or initial coats- 0f vitreous enaniel without the primary boiling defects heretofore encountered in a single or initial coats. Reference is hereby made to the teachings of that case, because; as hereinabove indicated, the

provision of iron or mild stecl stockcapable of receiving; a single or initialcoat of porcelain a ef ve the invention herein. In view of :4. Normalize.

3 such reference we shall not attempt to summarize the teachings of the copending application otherwise than to say that they include subjecting the stock or articles to a surface decarburization which is a brief, continuous heat treatment at temperatures between substantially 1250 and 1650 F. in an atmosphere, non-oxidizing to iron, and containing hydrogen and from 2% to 30% water vapor. This decarburization preferably follows such treatments as normalizing and the imposition of adhesion promoting coatings, e. g., coatings of nickel, when they are used, as well as the picklingor other, chemical treatments, wherever possible, to which the stock is subjected; and it has a number of effects: Among other things, it destroys gasforming residues, carbonaceous smudges and residues left by chemical treatments; it destroys massive carbides in the surfaces of the stock or articles as .well as lowering the carboncontent in. the surfaces to a value such that carbon is no longer effective as a gas producer during the firing'of vitreous enamel. Also, a suflicient lowering of surface carbon permits box annealing without "diffusion of interior carbon to the surfaces in amounts productive of trouble, and also permits subsequent light pickling without the formation of carbonaceous smudges.

An, exemplary routing, for enameling stockin accordance with that invention is:

. Hot roll the metal to thin bar.

. Pickle. n Cold reduce to gauge.

Electroplate with nickel.

. Decarburize.

Temper roll.

ther routings mayalso be employed.

. .The teachings of. the present application depend upon'the provision of a stock capable of acdefects which in the past have necessitated the manufacture .of enameled articles by processes involving plural. firings of separately fired-coats. ,We prefer a stock made in accordance with the pable of receiving aninitial fired coat of vitreous enamel without exhibiting the coating defects to which reference has been made.

I In our process it is necessary to make or select the two enamel substances in such a way that the firing temperature of the ground coat is no higher than, and is preferably lowerthan that of the cover coat. By firing temperature, we refer to the temperature at which an enamel becomes sufliciently fluid to flow on a metal sheet which has been prepared for enameling to form a smooth, glassy surface in an economically short length of time, such as three to five minutes on a 20 gauge sheet, when fired as a single coating of conventional thickness. Firing temperature is a characteristic of any given enamel, resulting from its particular compounding of ingredients, and is a property well understood by both enamelers and frit'producers. It is related to the fusion temperature range of the enamel, but is a more useful term, since any enamel fuses and changes fluidity over a rather wide range of tem- 4 perature, and since the extent of the enamel fiow is also a function of time.

A second requirement for the ground coat in our process is that it be very thin; and we employ ground coats having a fired thickness ranging from substantially .0004 inch to substantially .001 inch.

A third requirement is that the ground coat especially be milled or ground finely as hereinafter taught. A fine and uniform milling of the cover coat is also of advantage.

In our process, only about one-eighth to onefourth the former ground coat thickness is required. This permits consistent reduction of the total enamel thickness to .010 inch or less, with corresponding increase in resistance to chipping as the total thickness is lowered. This effect is icepting an initial fired coating without-the usual cover coat good appearance.

already appreciable at a thickness of .010 inch, and becomes increasingly marked as total thickness is reduced to the minimum at which the ground coat will give good adherence and the White coatings having good reflectance and appearance have been successfully produced as thin as .005 inch using commercial cover coat enamels, and still thinner coats, even as low as .003 inch can -be made by the same practice, using more opaque cover coat enamels, or using commercial enamels for applications not required.

The use of degassed stock or articles renders unnecessary the thicknesses of groundcoating heretofore found necessary to take care ofgas evolution and attendant troubles. It is only necessary to use sufficient ground coat to obtain the desired adhesion. While the thinnest ground'coat which will give adherence is preferred for maximum resistance'to chipping, a thin ground coat where high reflectance is in our process, and having a firing temperature lower than that of the cover coat, has a special effect in our process which will be set forth hereinafter.

A satisfactory ground coat is'one designed to give good adherence. The best'of such ground coats today contain, along with nickel, cobalt in small amounts, wherefore the coatings are deep blue in color.

Work is being done on ground coats of lighter or white color, and such coats are available for our invention. As the enameler will understand, the ground coat frit is milled with water to form a slip and then applied to the article in any suitable way, spraying being an excellent mode of obtaining uniformly-thin coatings. Similarly, the frit for the white or cover coat is milled with water, opacifier and clay to form a slip and is applied over the ground coat. Intermediate drying is not necessary and is not preferred, though it may be practiced if will not be masked by the subsequent white coat.

The firing temperature of the conventional ground coatings is 1560 to 1590" F. 7

Also, in the conventional practice, the subsequentwhite coat is usually about .006 to':.008

"inch in thickness after firing. When two white acoaess coats are imposed upon a ground coat, the: secnd one is about this. thickness, while the first one is usually reduced to a thickness of about 1004' to .005 inch. The firing temperature of conven tional whiter. cover coats. is about 1500. to 1520 F; or even. up to 1560* for some-acidpresisting enamels, which is. slightly lower than tha of the usualground; coat the conventional practice, the ground coat isapplied and fired at a. SUitab1Yhighp tune, after which the white: cover coat is. applied and fired at. a; somewhat lower temperature; the attempt being to applyand fire the. white. coat andcause it tomask the. imperfections in: the ground coatwhile. disturbing the ground coat as little as possible and while preventing excessive.diffiusionof'thetwocoats. Y 1 3 in the conventional practice", the white or light. colored cover coat is fired onto the: previousl'y fired ground coat at a temperature such that the; ground coat does not soften appreciably, and hence diffusion of the ground coat. into the cover coat is minimized. The: ground coat-is made to have suficient thickness to. fiow an'd re.- lease, bubbles produced by primary boiling; and while the/ground coat absorbsisomeoxide from the surfaces. of. the ferrous base, its 'thickness-is 'suclr that. its fusion temperature. is not: greatly affectedthereby. The-white or'lightcolored cover coat-is: made thick. enoughztormask; the imperious ti'ons of the. ground. coat due to the; primary boiliing,. .and it; generally: has: a. fusion: temperature definitely below that: of the .groundcoat: While .itmight be: supposed that a-qsayingin cost. would: be: obtained. if: both coats could be fired; at one; tima. attempts; to do. this hitherto have; proved. impracticall. With the combination of: enamels: described the firingot the "enamels at asufficiently: high; temperature tofuse the ground coatand obtain good adherence; would spoil, the gloss, reflectivity and iopacity of: the white; cover: coat. White cover coats. have. to be fired: attemperatures low enoughtaavoida tendencyrfor' the opaciiy-ing agent tcr dissolve excessively-imthe: glass,.ontranspaIency -a11d darken.- ingwilhresult; Furthermore.the;concurrent;fire ing of: a conventional: thickness; ground. and ofca cover coat; having? a. firing. temperature lower or everrthe; sameas; that of the;- ground. coat would resultin intermingling and diffiusiomof: the. two and the production of a defective; article: Yet again, with-the total thicknesses. of; enamel; hitherto current; in; the; art tha firing would-L give; rise to: flow and displacement; problems, since it: isfrequently the'practice'zto: fire the articles: with 501118101 all of their: enamel-coated surfaces. in thewerticakposition;

Given. a: degassed: stock;. the key; conceptwhich makes-it possibleifor us; to: fire simultaneously a ground and a; cover coat in. accordancezwith this. invention is, the; new relationship or firing temperaturesof; the: two coatings-i providingtth'ese coatings; are; thin; As; indicated, the-ground coat must? have a" firing; temperature not higher than, and. preferably-lower than; the; firingttemperature; oiethei cover: coat. In practice this; usually and preferably amounts; to arr'eduction. ofgpthe firing temperature of the-groundcoatzrather than amincreasein the firing temperature-of thecoyer coat. In .thepractice oi our. processitisbelieved that The an. entirely-different, mechanism occurs:

ground coat, as hasbeen-z indicated, has-a firing temperature. not. higher than and. preferably lower han hat-Q52 he rover s at.

6 coated. article isiraised temperature 'dtirhig the firing the ground coat is first melted. and

' attams that degree or fluidity whichenables it to make a good bond; with the base metaL. In doing so itabsorbsoxide from the base. metalsuriaces; but by reason. of the exceeding thinness of theground. coat: this absorption of oxide affectsv :it'ssoiltening point appreciably, and. the'ground coat, we. believe, almost instantaneouslysetsa' up to: a. condition. of very muclr. greater viscosity. During the initial. highly fluid condition oi? the ground coat, the cover coat, ifsof-tene'd was at least in. a. relatively' viscousz condition not tend:- ing toward. diftusion; By the time the cover coat has attained. its. maximum fluidity under thapar ticulanfiring cnnditions the groimdcoat w-ilii have attained a. higher viscosity-:againqtendingl to ore;-

' vent diffusion. We believ that. mechanism explains the surprising fact that diifusion. does not. destroy the appearance: of: our product despite the thinness .ofrour' coatings and the lower firing temperature of the: ground coat as. compared withxthe-covencoat." 1

For; example; we may: use a; conventional-white cover coatin a thickness of' .006 to .OOBinchg. and hawinga; firing temperature; of. 15.00% to ,15602? F: Withthis; howeven. weyusez a. blue, ground: coat having. a firing. temperature of about 1400: to 1500?: F; The two: layers are fired. simultaneously atthefirin ;temperatureaoffthecovercoatl With such; a combina i weobtainzgood. bo d without lowering; the;glossv or reflectivity: of; the: white; coat ondegassedstockai 1 r v I The thickness of the ground; coatingfisfprefere ably reduced atthe -same time itsfiringtempwatureis lowered. has been. explained; the use of; de assed}. s o k-Mak s it unn c ssarrto: m: ploy heav gr nd oat. and. the: advanta es of:-

over-all v enamel; surfacing, can; be real.- ized. Our; preferred; surfacing; is made; up; ofia ground: coat; witha firing. temperature between s bs ant a ie :4 0 and of suc wei ht: as d fifi fi edi y s b t in h: hi k-r. .1. less, and. a white covercoat with a firing tern-,- n ra ure bs antia l 15 5 fi lwh weightthatit williorm ale-yer about; 0065130; .008 h: f e fi in ot a rs. ei g: simu taneo sly fired, atthefiring, temperatureofi the cover coat; r n h-l z imezp o r to seth nameh which is of the-"order of: to; 5- minutes; n -.20 augemet ior x mn e- With such: a combination, we; obtain imone firi a dhesiom e hefl ct v tr; in: creased chippingv resistance; and a good. white color. The thinner ground coatingiis, moreeasily fir d. iveod d r e: i h u d m -s ne.

the coyercoat just-because of its: smaller mass,

'Wehave notednotendency for. the two coatings ter; and ,in particular-theme off cover coats ('desirableforsome: other. reason; such. as opacity).

of suchpooradherence to; iron. and; steelasgtrr bQimPDSSiDIQ Of11561852; first coat. I

The-firing temperatureof tha roundcoa'tmust.

not be higher than the firing temperaturaof the;

0 peratureaofgtheground coat from- 20i=to=100f."E:

-,-. High adhesionground coats and cover coats of white or light colored enamel constitute well :recognizedclasses of enamel substances familiar to the, enameler and readily'ascertainable' from standard texts; For example, reference is made to .thepublication entitled Enamels by Andrew :I. ,:A'ndrews,. first edition,. published. 1935 .by the Twin: City Printing Company. Beginning at page =l65, this publication has an extensivesection on ground coatsv and their analyses, pointing out that there are whiteand gray groundcoats as sieve-opening is .0029 inch. The fineness of sub :well as the-more common cobalt blue ground coat and giving analyses of variousground coats. A .conventional cobalt blue ground coat .contains feldspar,v borax, quartz, soda. ash, soda nitre,

fiuorspar, cobalt oxide and manganese dioxide. The firing. temperatures of ground coats depend upon their relative contents of such materials as borax', :feldspar and quartz, generally expressible in triangularicomposition diagrams, such as those tof-be found in the above publication on pages 1'72, 1'73 and 174. For example, thegreater the quantity of quartz, the higher the firing temperature will'be, while an increasein the quantity of borax will lower'the firing temperature. Thus it is well understood bythe skilled worker in the enameling art howthe firing temperature offany particular type of ground coat canbe raised or lowered by controlling the relative percentages of its ingredients. Our process isapplicable to all o'f the 'gr'ountcoat compositions set forth in the publication above noted; without exception, as well as to all ground coat compositions we have encounteredinpractice. The same publication; beginning at page '178, has a section on white cover enamels- In general, these consistof such materials as feldspar, borax, quartz, soda ash, soda nitre, fluorspar and cryolite. The metallic ingredients designed to promote adhesion, and which ordinarily darken the color of a ground coat, are omitted in white-or light colored cover coat enamels; and an opacitying agent is added to give the desired opacity and color. There are a number of such 'opacifying agents and we mention without limitationsuch exemplary ones as antimony oxide, titanium oxide, tin oxide, zirconia, and the likef A section on acid resisting enamels begins on page 182 and a section on colored'en'amels o page 1-85 of the above entitled publication.

Again, in the case of ground-coats, the firing temperature may be controlled by controlling the relative proportions of oertain'ingredients, usually borax, feldspar and quartz; The relationship of these'may'be expressed in a tri angular diagram, such as that shown cn'page 182 of the publication. The same considerations ap ply as in the case of ground coats. In the practice of our invention we may employ any of the White'or light. colored co'ver coat compositions, without exception, set forth in the said publication and any others of which We are aware] The above publication also contains a section beginning at chapter 12, page 325, on the properties ofenamels, giving various tests by which their thermal behavior may be determined, ill-.1 eluding their fusion temperatures as set forth in this specification. r 1 I In the practice of our process wehave found it advisable to grind the fritsfinely, and we prefer to employ both ground coats and cover coats ofrsu'chfineness that they will pass through a 200-mesh sieve, as set forth on page 384 of the above entitled publication; In'such a sieve having 200 meshes per lineal inch, the size of the division of the frit makes forsmooth, uniform and continuous coatings and tends to prevent diffusion, inasmuch as larger particles may result in some actual interlocking of the particles inthe applied coatings in a single fire process.

. .The thicknesses of the coatings given hereinabove are the fired thicknesses. It will be understood that finely ground frits dispersed in water are normally appliedby means of aspray gun, and control of thethickness of a layeris usually accomplished by spraying a given weight of the solids on a given area of the surface to be enameled. Upon firing this .will. providea coating of given weight andhence ofv determined thickness.

We are not restricted to the simultaneous firingof a. total of two superposed coats of porcelain enamel. In addition to a ground coat and a cover coat," we may employ oneor' more intermediate coats for various purposes. The'number of coats simultaneously firedis" not a-limitationupon our invention, keeping inmind the following matters. The firing behavior of enamel is not only a function'of temperature but also'of time, which is rather critical. The firing must belong enough to develop bond and strength, whether the enamel be attached to the base metal or to an underlying coat of other'enamel. But thetime must be short enough to prevent the loss of opacity due to solution of opacifier in the glassy matrixand due todiffusion of iron'oxide from the 'basemetalinto the enamel. I While the time interval between the fusion of'th'e surface and the fusion of the interior of even a thick layer of enamel is short, it is neverthelessappreciable. Even a single white coat cannot be put on too thickly or opacity may be lost by an overfiring of the surface before the proper maturing and bond'of the underlying portion of the enamel is achieved. Inview of theseconsiderations there is a maximum total thickness of enamel which shoul'd'be fired at any one time. Under ordinary conditions, if the. total thickness of. the enamel layers employed exceeds around .015 inch, overfiring of the surface is likely to result. For this and for other reasons which have been given above, it is preferred that the total or combined thickness of all layers of enamel to be simultaneously fired be not over .010 inch.

The greater the differencebetween the firing temperature of the ground coat and that of the cover'coat within practical limits, the greater can be the total or combined thickness of all coatings to be simultaneously fired. While excellent results can be attained with a ground coat and a cover coat of the same firing temperature if the layers are kept-quite thin, difliculty is frequently encountered at total thicknesses approaching .015 inch, whereas the thicker coats can readily be fired if the ground coat has a substantially lower firing. temperature than" the cover coat. Similarly in simultaneously firing more than two coats, it is well to have a firing temperature gradient as between the coats, those lying inwardly of the surface having progressively lower firing temperatures.

Modifications may be made in our invention without departing from the spirit of it. Having thus described our invention in certain exemplary embodiments, what we claim as new and desire.

tosecure by Letters Patent is: l.;In a process of producing smooth, continuous, light colored porcelain enamel coatings on stock having its surfaces ferrous enameling sheet stock free from gas forming substances productive of pits and spots, the coatings having a high degree of adherence to the stock, the steps of imposing upon the surface of said stock a thin continuous coating of high adherence ground coat enamel of such thickness that after firing it has a thickness substantially between the ground coat has a firing temperature 'substantially 20 to 100 lower than the perature of the cover coat.

3. A process of making a firing temsingle fired white or light colored enameled article having good .0004 and .001 inch, ap-' plying at least one subsequent continuous,coat- 10 ductive of gas upon firing, and a single fired plural layer coating of porcelain enamel comprising a high adhesion groundv coating next the surface of the ferrous material and an overlying light colored cover coating, both of vitreous enamel, the ground coating having a thickness to give, upon firing, a layer substantially between .0004 in. and .001 in. in'thickness, the firing temperature of the ground coating being not higher than that of the cover coating, the total thickness of said fired coatings being substantially from .003 to .010 inch.

'7. A single fired white or light colored enameled article having low cost, high gloss, reflectivity and opacity, comprising a base of ferrous material having surfaces free of substances productive of gas upon firing, and a single fired plural layer coating of porcelain enamel comprising a high adhesion ground coating next the adhesion, which comprises imposing upon the surface of ferrous material freed of substances productive of gas and oxide spots upon firing, a ground coat of such thickness that after firing it will have a thickness substantially between .0004 in. and .001 in. and of high adhesion vitreous enamel substance having a firing temperature of between substantially 1400 to 1500 F. and a cover coat of vitreous enamel substance of such thickness that the total thickness of both coats after firing will be substantially .003 in. to .010 in. and having a firing temperature of between substantially 1500 to 1560 F., and firing both coatings at the same time, at the firing temperature of the cover coat.

4. A process of making a single fired White or light colored enameled article having good adhesion and enhanced chipping resistance, which comprises imposing upon the surface of ferrous material freed of substances productive of gas and oxide spots upon firing, a ground coat of high adhesion vitreous enamel substancehaving a firing temperature of between substantially 1400 to 1500 F. and a cover coat of vitreous enamel substance having a firing temperature of between substantially 1500 to 1560 F., and firing both coatings at the same time at the firing temperature of the cover coat, the weight of ground coat so applied being such as to produce a fired layer of substantially .0004 to .001 inch in thickness and the total weight ofboth coatings so applied being such as to produce a total thickness of fired enamel of substantially .003 to substantially .010 inch.

5. The process claimed in claim 1 in which the ferrous enameling stock-is a ferrous sheet decarburized to the extent of being freed of carbonaceous smudges, massive carbides andgas producing carbon.

6. A single fired white or light colored enameled article having low cost, high gloss, reflectivity and opacity, comprising a base of ferrous material having surfaces free of substances pro surface of the ferrous material and an overlying light colored cover coating, both enamel, the firing temperature of the ground coating being not higher than that of the cover coating, the total thickness of said fired coatings being substantially from .003 to .008 inch, the said ground coating being a blue, cobalt-containing coating and the said cover coating being a white coating of high reflectance.

8. A single fired white or light colored enameled article having low cost, high gloss, reflectivity and opacity, comprising a base of ferrous material having surfaces free of substances productive of gas upon firing, and a single fired plural layer coating ofporcelain enamel comprising a high adhesion ground coating next the surface of the ferrous material and an overlying light colored cover coating, the ground coating having a thickness to give, upon firing, a layer substantially between .004 in. and .001 in. in 7 thickness, the firing temperature of the ground coating being not higher than that of the cover coating, the total thickness of said fired coatings being substantially from .003 to .008 inch, the said ground coating being a blue, cobalt-com taining coating having a firing temperature substantially between 1400 to 1500 F. and the said cover coating being a high reflectivity, white coating having a firing temperature substantially between 1500 to 1560" F.

THEODORE F. 0LT.

JESSE J. CANFIELD.

asrunnncss CITED 1 The following references are of record in the file of this patent:

UNITED STATES PATENTS of vitreous 

1. IN A PROCESS OF PRODUCING SMOOTH, CONTINUOUS, LIGHT COLORED PORCELAIN ENAMEL COATINGS ON FERROUS ENAMELING SHEET STOCK FREE FROM GAS FORMING SUBSTANCES PRODUCTIVE OF PITS AND SPOTS, THE COATINGS HAVING A HIGH DEGREE OF ADHERENCE TO THE STOCK, THE STEPS OF IMPOSING UPON THE SURFACE OF SAID STOCK A THIN CONINUOUS COATING OF HIGH ADHERENCE GROUND COAT ENAMEL OF SUCH THICKNESS THAT AFTER FIRING IT HAS A THICKNESS SUBSTANTIALLY BETWEEN .0004 AND .001 INCH, APPLYING AT LEAST ONE SUBSEQUENT CONTINUOUS COATING OF WHITE OR LIGHT COLORED COVER COAT ENAMEL OF SUCH THICKNESS THAT THE TOTAL THICKNESS OF ALL SAID COATS AFTER FIRING IS SUBSTANTIALLY .003 TO .010 INCH, DRYING, AND FIRING SUCH COATS SIMULTANEOUSLY AT THE FIRING TEMPERATURE OF THE COVER COAT, THE FIRING TEMPERATURE OF THE GROUND COAT BEING NOT HIGHER THAN THE FIRING TEMPERATURE OF THE COVER COAT. 